The present invention relates to silos for mixing stored granular or powdery materials, such as raw meal for cement. The invention is particularly adapted for use with a large mixing silo combined with a small homogenizing silo to produce thorough homogenization at a relatively low cost.
In the cement industry it is frequently desirable for a manufacturer to guarantee the calcium carbonate percentage of his product. However, the calcium carbonate content of a truckload of raw material from the quarry may vary 15% or more from the desired figure, with these variations slowly changing from truckload to truckload from too much calcium carbonate to too little. The variations may be reduced to the 5% range during preliminary processing at the cement mill, but final mixing is generally necessary in order to reliably achieve the guaranteed percentage of calcium carbonate.
Mixing silos have long been used in the silo art due to their modest energy requirements. A stream of incoming nonhomogenious material is poured into the silo, forming layers of differing consistency at different vertical distances from the silo bottom. Lateral inconsistencies from one side of the silo to the other may also form. When a discharge aperture at the bottom of the silo is opened the stored material rushes out, the stored layers sagging into a cone-shapped depression above the opened aperture. When the layers sag they also thin, so that a given quantity of material collected at the discharge aperture contains samples of more layers than would have been received had the same quantity of material been scooped up by a workman who climbed into the silo. Additionally, the discharge process creates turbulence which enhances the mixing effect.
Nevertheless, the performance of the mixing silo is relatively poor. The layer-thinning effect of a discharge aperture is most pronounced for the material directly above it, and material to either side participates inadequately in the mixing process. There is also the problem that a cone might grow too deep, so that incoming material might "punch through" to the discharge aperture without undergoing much mixing. A number of workers in the art have tried to enhance the performance of mixing silos via a variety of expedients, including providing different discharge apertures at different positions on the silo floor around the periphery of a conical hood, but the fact remains that the performance of mixing silos is inadequate for many purposes.
Homogenizing silos are also well known in the art. Basically, material is stored in an homogenizing silo and air is blown through it to create turbulence which thoroughly mixes the stored material. The silo should be filled to a proper level and not above, since an adequate empty space or head room must be maintained in order to ensure proper mixing. After mixing has been completed, the homogenized material is removed for use and a new batch of nonhomogeneous material replaces it. Homogenizing silos typically have a large capacity since it is desirable to include material having the entire range of variations within a single batch. The mixing performance of a large-capacity homogenizing silo is excellent, but vast amounts of compressed air are required for proper performance. This compressed air is becoming more expensive as energy supplies dwindle. Moreover, the necessity for batch processing is frequently disadvantageous. The energy input requirements for mixing silos are considerably less severe, since mixing is produced by gravity rather than compressed air, and additionally mixing silos offer the convenience of continuous processing. Nevertheless, as mentioned above, the performance of mixing silos is inadequate for many applications.
Applicant's parent application disclosed a relatively large mixing silo having a conical hood at the base thereof and discharge apertures spaced around the periphery of the conical hood, the discharge apertures being connected by conduits each provided with a flow control gate to a relatively small homogenizing silo positioned beneath the conical hood. The flow control gates are sequentially opened to allow material to flow into the homogenizing silo until a level recorder mounted therein signals that the material level for proper mixing has been achieved. The application mentioned the possibility of opening two flow control gates simultaneously and varying the rate at which material is discharged through them, but did not disclose how to do so.